A: Hastelloy bolts are definitely ideal in this concentration of hydrochloric acid. Hastelloy C276 bolting is usable in these temperatures and concentrations, but can’t handle the yield strength you need. Hastelloy C276 has a yield around 49 ksi – about half your requirement. For your situation, you have two choices. Incoloy 925 bolts have similar corrosion resistance to alloy 825 which can take the HCl, yet they are age hardened to offer a lot higher strength than 825. Alloy 925 bolts have a yield around, 120ksi. In addition, you can also try Inconel 718 bolts which are an age hardened form of 625. Therefore it offers similar corrosion resistance but has a yield of 175ksi. Both will still show slight corrosion over time but will be the best options considering you high strength requirements.

Q: I’m operating a FGD (Flue Gas Desulphurization) plant and am experiencing some mild corrosion on my scrubber bolting. I’m currently using Incoloy 825 bolts. It’s mild corrosion but non-the-less I’d like to have it cleared up before I get additional issues. Any suggestions?

A: Since scrubber liquids and gases not only contain sulfuric acid but also chlorides, you have double trouble. Incoloy 825 bolts are great for some parts of flue gas desulfurization systems but sometimes not where there is scrubber slurry because its so potent. Here you may want to try Hastelloy C276 bolts which have excellent resistance to sulfuric acid as well as the ability to withstand higher chloride content than other alloys before the onset of localized corrosion in a simulated scrubber environment. Corrosive conditions are also amplified on shutdowns when acid gasses could condense on corrosion resistant bolting.  I’ve also posted a chart for you which compares steel, Incoloy, Inconel and Hastelloy in terms of flue gas scrubber corrosion.

Q:  I am considering using Inconel 718 fasteners with a Monel K-500 flange in seawater. I’m concered as Inconel is the more anodic of the two and is also has a smaller surface area. Would this cause galvanic corrosion?

A:  This is a great question. In this case if you plan on using Inconel 718 fasteners with Monel flanges you should be OK. Because the Inconel is the more noble, the galvanic effects will be pushed to the K500 Monel. This is where you want any galvanic effects to be on the larger mass part and not on the fasteners. Because the mass of the flanges is much larger than the bolts, the galvanic effects will be minimized.  Furthermore the galvanic effects between Monel and Inconel as relatively minor.  See our galvanic compatibility chart

Q: My customer is having an issue with his stainless steel fastener bending. They are being used in a high-heat application running at approximately 1600F. Is this normal? Any ideas?

A: This is definitely expected at these temperatures. If temperature and strength are you main focus, I would suggest either Inconel 600 screws or Alloy 330. Inconel is known for it’s high temperature strength offering a yield strength of 84ksi and a usable temperature limit of about 1800F. Inconel’s could used in air or in a vacuum.  Alloy 330 is another alternative as it has very similar high temperature properties but would be best suited inside a vacuum to minimize oxidation.

Q: I have a high temperature application which is exposed to sulfur and chloride gasses. We’re operating at between 1200° and 1400°F. Any suggestions on a bolt material to consider?

A: To deal with both sulfuric/chloride atmospheres as well as high temperatures, you may want to consider Incoloy 800HT bolts or an Inconel 600 bolts depending on the concentrations of the acid gasses. Incoloy is an ideal material for long-term exposure in high temperature environments due to its oxidation, carburization and creep resistance. Incoloy 800 series bolts are all similar and considering your temperature range, I would suggest the Incoloy 800HT since it will not become embrittled even after long periods of usage in the 1200-1600° F. However if you have more severe corrosion issues especially during shut downs or equipment cycling due to acid gasses or condensation formed by the sulfur and chlorine components combining with water, the Inconel fasteners will provide better corrosion resistance than Incoloy.

Q: Inconel 625 and Hastelloy C276 bolts seem to have similar Nickel content. Does this mean these fasteners will perform the same in a corrosive environment?

A: Inconel 625 and Hastelloy C276 bolts do offer similar corrosion resistance but Hastelloy C276 has a slight edge in many acidic environements, especially when fasteners are in hydrochloric (HCl) and sulfuric (H2SO4) acids. Though nickel is critical to corrosion resistance, molybdenium and chromium also play a role in this. Chromium provides resistance to oxidizing chemicals, whereas the high nickel and molybdenum contents supply resistance to non-oxidizing environments. Inconel 625 bolts have a higher chromium content than Hastelloy C276 bolts which provides it an edge in oxidizing environments, whereas Hastelloy C276 as a higher Molybdenum content which makes slightly it more resistant to acids than Inconel 625.

Q: I’m working with boiling sulfuric acid and tried Alloy 20 for some of my bolting but am having corrosion. I’m confused as I thought that is what they are best for. Any suggestions.

A: You are right Alloy 20 does typically perform very well in sulfuric acid and even boiling sulfuric acid but only for a specific range.Not knowing your specific situation, it is most likely the concentration of acid you are working with that is the issue. Alloy 20 provides great protection in boiling sulfuric acid at specific range of 20% to 40% concentration. If you are over this concentration, even at 45%, if may cause corrosion issues. For high concentrations of sulfuric acid, you may want to look at Inconel 625 or Hastelloy C276. Both Inconel 625 and Hastelloy C276 bolts should provide you protection in all concentrations of sulfuric acid up to 80%. For anything higher than 80%, I would suggest Tantalum which can handle all concentrations and temperatures.

Q: We are running a steel pickling process are considering switching from a sulfuric acid process to hydrochloric acid. Our system currently uses Inconel 625 bolting. With this be sufficient?

A: Inconel 625 is a good material for acid corrosion but for large system of hydrochloric acid, especially with the corrosion that can occur with fumes and vapors, you may want to try a Hastelloy C22 or even tantalum if the Inconel 625 does not hold up. Both perform very well in hydrochloric acid. Tantalum fasteners have the best corrosion resistance and are essentially immune to HCl but limited strength. Hastelloy especially grade C22 while not as corrosion resistant as tantalum has very good corrosion resistance to HCl and good strength as well.

Q: I’m using zirconium fasteners in a sulfuric acid process but they are showing corrosion. I’m not sure why as I’m well below boiling and only using 40% concentration. There may be some downstream contaminants, but not much. Any ideas?

A: It is most likely the contaminants that are causing your zirconium screws to corrode. Though zirconium can easily handle sulfuric acid well beyond boiling and at 70% concentration – a far harsher condition than you have – it also has an Achilles heel which is oxidizing media such as ferric or cupric chlorides. Even if a minute amount of oxidizers are seeping into your process where they zirconium screws are, you will have corrosion. Depending on the exact temperature that you are running at, I would suggest trying Hastelloy C276 or Inconel 625 if your process is 150°F or lower. Both are similar in that they only have 1mpy of corrosion at 150°F in 40% H2SO4. Inconel is traditionally higher strength and Hastelloy with a slight edge on corrosion resistance. If you have a lot of oxidizing contaminants, you may want to look at Hastelloy C22 which offers <1mpy under the same conditions, but has a much better resistance to oxidizing media that C276. For higher temperatures – nearer boiling, tantalum may be the answer.